Switching-mode power controllers (e.g., inverters, converters, switching-mode amplifiers) are used when voltage or current into a load is to be controlled with minimum loss of power of the controller. Frequently with a.c. outputs there is a desired output waveform. In d.c. to a.c. inverters, for example, it may be desirable that the output voltage into a load closely approximate some reference voltage, possibly sinusoidal. When the output is synthesized by a sequence of rectangular pulses as with pulse-duration modulation for example, the fidelity to which the output waveform conforms to the desired waveform depends on many factors, such as the frequency of switching, the particular modulating scheme chosen, the physical limitations of the switching devices, and the characteristics of any incorporated output filtering.
When the output signal from the switching mode power controller must span a relatively large dynamic range, such as might be necessary when the reference signal is a music or voice signal, the required output pulses from the power switches become very short in duration at the low power levels. In the past the ability to make pulses of short duration and therefore the ability to cover a large dynamic range has been limited by the finite rise, fall, and recovery times of the power switches themselves.